1. Field of the Invention
This invention relates to the field of electrodeposited copper traces. More particularly, this invention relates to the field high strength electrodeposited copper signal traces for hard disk drive suspensions.
2. Description of Related Art
Typical magnetic hard disk drives for storing large amounts of data include a transducer mounted on a slider for reading and/or writing the data on the disk. In such systems, the slider is typically attached to an actuator arm by a suspension system, which includes both a load beam and a gimbal typically made of stainless steel. The flex trace suspension further includes the copper signal conductors which carry the data signals to/from the transducer, carried on a dielectric material such a polyimide which electrically isolates the signal traces from the stainless steel load beam. The copper signal conductors are defined upon the composite structure comprising the copper signal conductors, the insulating polyimide dielectric layer, and the stainless steel layer. The stainless steel layer provides both mechanical support and an electrical ground reference plane in this microstrip configuration.
Both additive and subtractive manufacturing processes may be used to build suspensions. Subtractive processes begin with preexisting laminates of stainless steel, dielectric, and copper. The laminates typical begin by depositing an alloy copper onto a drum. The resulting copper foil is then removed from the drum and laminated with a dielectric material and stainless steel underneath to form the composite structure which is then selectively patterned and etched to form the suspension including the signal conductors. Subtractive processes are well known in the art.
Early subtractive suspension manufacturing processes such as defined in U.S. Pat. No. 5,955,176 issued to Erpelding, et al. utilized rolled alloy copper foil C17510 which was laminated into a composite. Later, circuits transitioned to NK120 & C7025 alloys which possessed similar mechanical properties but exhibited approximately 20% improved conductivity and did not contain the hazardous substance beryllium. Alloy foils can also be strain- and precipitation-hardened to increase their mechanical properties.
To meet emerging density and signal propagation speeds, in more recent years manufacturers have increasingly relied on the additive method of manufacturing suspensions. This is because electrodeposited copper can generally be made thinner than the rolled copper of suspension laminates, thus producing very thin copper traces having the lower pitch and roll stiffness which is required for very small sliders.
Additive processes for making suspensions are well known in the art. In an additive process, various layers including dielectric and then copper are deposited on the stainless steel substrate serially. In a typical additive manufacturing process a polyimide dielectric layer is first cast onto the stainless steel and cured at high temperature. An adhesion layer is then sputtered onto the polyimide. The purpose of the adhesion layers is to enhance the bond between the polyimide dielectric and the copper signal traces. The adhesion layer may be, for example, Cr as disclosed in U.S. Pat. No. 4,863,808 to Sallo and U.S. Pat. No. 6,489,035 to Wang, or Ti or Monel as disclosed in U.S. Pat. No. 7,379,271 to Schreiber et al. Copper is then typically sputtered onto the adhesion layer, followed by electrodeposition of a copper layer to form the desired signal traces, a Ni layer for adhesion, and finally an additional polyimide layer for electrical insulation. Typically, the copper that is electrodeposited to form the signal traces is essentially pure copper and can be very thin, such as less than 10 μm. Other variations on, and refinements to, the basic additive process, including various cleaning and preparation steps, will not be discussed in detail herein because they are not considered relevant to the present discussion.
The suspension is an electromechanical system in which both the electrical and mechanical properties must be balanced. Mechanical properties are extremely important; the properties of spring rate, static attitude angle pitch, and roll stiffness must be stringently controlled to allow the read/write head to accurately fly over the recording media. Typical spring rates are in the range of 20 N/m, typical pitch and roll stiffness are in the range of 50 μNM/degree, and typical static attitude adjust angle tolerances for pitch (longitudinal axis) and roll (transverse axis) for today's head gimbals is 0.20 degrees. The copper trace should be thin for mechanical purposes (typically less than 10 μm) yet not prone to creep, and yet still carry electrical signals efficiently. The copper trace should preferably have a rectangular cross section rather than a trapezoidal cross section for increased routing density, and should have clean edge lines rather than be dendritic, and have smooth surfaces, for good electrical performance.
During manufacturing, fine adjustments are made to the pitch and roll static attitude of the gimbal and slider using mechanical and/or laser induced micro-bending. The main focus of the bending is usually to the outer arm portions of the suspension that are sometimes referred to as the out-rigger and in-rigger portions; however, the copper conductors of the suspension often experience enough stress during the static attitude adjustment so that they mechanically yield. If the copper yields, it will undergo stress relaxation (creep), and the precise suspension adjustment will creep or drift over time. Drift results in the air bearing slider performing poorly such as by flying higher than desired, which results in degraded reading and writing of the data to and from the disk.